power: fg-alg: Add support for OCV based step charging in TTF algorithm

Currently TTF algorithm support Vbatt based step charging, but since
oscillation in Vbatt is high when the load changes abruptly, we have
moved to OCV based step charging. Add support for OCV based step
charging in TTF algorithm.

Change-Id: Iac2b6e694ef7b93cf7cedd237752d1eea0cb8ece
Signed-off-by: Sahil Chandna <chandna@codeaurora.org>